Unsaturated resin compositions are finding increased use in the automotive industry in SMC formulations from which component parts, especially body panels, can be molded. The unsaturated resin compositions contain, in addition to the unsaturated polyesters and monomer components, so called "low-profile" additives which are thermoplastic polymers that act to prevent undesirable shrinkage as the composition is being molded into a thermoset article. Of particular note recently has been efforts to improve the impact resistance of exterior automotive body panels. One skilled in the field knows that polyester body panels display quite brittle behavior in impact tests relative to traditional materials (e.g. steel). The cause of the low impact resistance generally lies in the dual performance required for these materials: superior reproduction of the mold surface (excellent surface aesthetics) in addition to the usual stiffness and strength requirements typically requires the formulator to employ highly reactive unsaturated polyesters in addition to high amounts of thermoplastic (low profile additive).
The formulators also know that blending rubbers with thermosets may improve impact resistance by absorbing the energy of impact and delaying the onset of catastrophic failure. However, adding certain hydrocarbon rubbers to polyesters appears to degrade impact resistance. In the example of simply blending poly(butadiene- co-acrylonitrile) rubber and unsaturated polyester, incompatibility between the rubber and polyester leads to phase separation on a macroscopic scale. Thus, if one blends polyester with rubber and then immediately cures it, the rubber particles apparently assume a very large size and act as failure sites. Further, it is not possible to preblend rubber and polyester for later molding since during storage phase separation occurs between the components and usually gives a rubber phase above the polyester phase. In addition, when rubber is employed to improve impact resistance, adhesion, etc. of polyesters, the useful upper service temperature (HDT)significantly decreases. This is due to the lower glass transition temperature of the rubber relative to the pure thermoset.